SYDNEY (Reuters) - U.S. Defense Secretary Jim Mattis said on Monday the United States and its allies would continue to fight against Islamist extremists and would not be scared by attacks against the West by the Islamic State group.

British Prime Minister Theresa May has said Britain must be tougher in stamping out Islamist extremism after attackers killed at least seven people by ramming a van into pedestrians on London Bridge and stabbing revelers in nearby bars.

Islamic State, which is losing territory in Syria and Iraq to an offensive backed by a U.S.-led coalition, said its militants were responsible for the attack, the group's media agency Amaq said in a statement monitored in Cairo.

One French national and one Canadian were among those killed. At least 48 people were wounded in the attack, including at least one Australian.

"We are united, as I said, in our resolve, even against an enemy that thinks by hurting us they can scare us," Mattis said, appearing in Sydney alongside U.S. Secretary of State Rex Tillerson for the first time outside the United States.

"Well, we don't scare," Mattis said.

Mattis and Tillerson were due to hold a media conference with their Australian counterparts later on Monday.

Australian Prime Minister Malcolm Turnbull said the world would likely see more such attacks, and that it was a problem that the global community must be "vigilant and determined and defiant".

"These criminals, these terrorists, are cowards. There is nothing heroic in what they do," he told an earlier media conference in Sydney.

"Now, this is a corruption, a disease, within Islam. It is a global phenomenon and it has to be dealt with globally," Turnbull said.

The three assailants in the London attack on Saturday were all shot dead by police. It was the third Islamist-inspired attack in the United Kingdom in the past three months, prompting British Prime Minister Theresa May to declare overnight that "enough is enough".